One of the major problems arising in the use of centrifugal gas compressors for applications where the compression load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller and diffuser passage must be sized to provide for the maximum volumetric flow rate desired. In centrifugal refrigerant compressors, the loads typically vary over a wide range and they may be operated at such low flow rates that their inlets and diffusers are too large for efficient operation. When there is a low volumetric flow rate through such a compressor, the flow becomes unstable. As the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, flow in both the impeller and the diffuser becomes separated from the wall along the entire length of the flow passage and there appears to be a partial reversal of flow in the diffuser passage creating noises and lowering the compressor efficiency. When the mass flow is decreased further, the compressor encounters what is known as surge, wherein there are periodic complete flow reversals in the compressor destroying the ability of the machine to do useful work.
Various techniques have been used to increase the range between the surge and choke limits of a compressor.
Guide vanes located in the inlet of the compressor have been employed to vary the flow direction and quantity of the entering gas since the work done by an impeller is proportional to the difference of the square of the gas velocity at the impeller exit and the impeller inlet. Inlet guide vanes improve efficiency because they impart a swirl to the gas at the impeller inlet in the direction of rotation thus reducing the velocity difference. The lift capability of the compressor is also reduced, but for normal air conditioning applications this is no problem because the required lift also falls off as load decreases. Sometimes, mechanically connected to these guide vanes is movable diffuser structure to throttle the diffuser passage as the inlet flow is reduced.
As evidenced by U.S. Pat. No. 5,437,529, free rotors (freely rotating rotors) may be used as a part of the apparatus used for the dynamic control of rotating stall and surge in turbo machines such as compressors. The device of this patent has the limitation that its effectiveness is dependent upon the speed of rotation of the freely rotating rotor, which, in turn, is proportional to the volumetric flow rate. Therefore, when the compressor operates in the low flow regions of the compressor map (region in which the flow is less than design flow), the free rotor slows down and its effectiveness is reduced. Unfortunately, achieving a given amount of effectiveness in the low flow region results in high free rotor speeds when the compressor is operating at the design condition. High free rotor speeds increase the manufacturing cost and increase the pressure loss across the free rotor, thereby reducing its practical implementation.
The effectiveness of the free rotor described in U.S. Pat. No. 5,437,529 is expressed as .phi. (tan .gamma.).sup.2, where .phi. is the flow coefficient (defined as the ratio of the gas axial velocity component at the compressor inlet to the impeller tip speed) and .gamma. is the stagger angle for the free rotor blades. As the compressor mass flow is reduced, .phi. reduces and, hence, the effectiveness of the free rotor reduces as well.